1. Field of the Invention
Pearlescent pigments that contain titanium dioxide in the top layer or that are made up of particulate TiO2 have a degree of photocatalytic activity. If UV light acts on a pearlescent pigment in the presence of water and oxygen, the UV activity of the pearlescent pigment can trigger an accelerated decomposition of organic compounds, e.g. of a binder matrix. The proportion of UV contained in daylight can already cause this reaction, i.e. for applications such as automotive coatings which are directly exposed to the weather specially stabilized pearlescent pigments must be used. In order to counteract this adverse effect for outdoor application, pearlescent pigments can be provided with various protective coatings to reduce photoactivity. Starting from aqueous metal salt solutions, poorly soluble compounds are usually precipitated as metal oxides onto the surface of the pigments. Two different metal oxides are predominantly used here. In order to promote the compatibility of the pigments with different coatings, but in particular with the more environmentally friendly water-based systems, as a rule a further additional organic modification is made to the top layer, e.g. by means of silanes.
2. Description of Related Art
According to the teaching of EP 0 632 109 A1a three-layered protective layer is applied to a platelet-shaped substrate coated with metal oxides. In a first step an SiO2 layer is applied, in a second step a hydroxide or hydrated oxide of cerium, aluminum or zirconium is applied and in a third step at least one hydroxide or hydrated oxide of cerium, aluminum or zirconium and an organic coupling reagent are applied. This three-layered structure is disadvantageously very laborious and correspondingly cost-intensive to produce. In addition the coupling reagents have to be hydrolyzed before binding to the pigment surface, wherein however, according to the teaching of EP 0 888 410 B1, only a maximum of 60% of the added coupling reagents can be bound to the pigment surface.
EP 0 888 410 B1 discloses modified pearlescent pigments based on a platelet-shaped substrate coated with metal oxides. According to the teaching of EP 0 888 410 B1, the top layer consists of at least two oxides, a mixture of oxides, or mixed oxides of silicon dioxide, aluminum oxide, cerium oxide, titanium dioxide or zirconium dioxide and a water-based oligomeric silane. The composition of the top layer is thus also very complicated and correspondingly laborious to produce.
EP 1 682 622 B1 also discloses a top layer of two metal oxides, wherein here first a cerium oxide layer must be precipitated and subsequently an SiO2 layer. Silanes are likewise predominantly used as coupling reagents.
EP 0 881 998 B1 discloses weather-resistant pearlescent pigments with a top layer either made of aluminum oxide or again of a two-layered structure of aluminum oxide and cerium oxide as well as silanes as coupling reagents.
EP 1 727 864 A1 discloses weather-resistant pearlescent pigments with a top layer made only of SiO2. However, these pigments are not always completely weather-resistant in all applications, in particular in the case of optically very high-quality pigments.
0 141 174 B1 discloses weather-resistant pearlescent pigments with a top layer which contains cerium hydroxide. In this document it is proposed that this top layer be supplemented by a silicate layer and preferably by further oxide layers, such as aluminum oxide or zinc oxide, in order to guarantee a better binding of polymeric siloxanes which can function as coupling agents.
In the above-named state of the art, the UV activity of the highly refractive TiO2 layer is usually suppressed by at least two different oxide layers or one mixed layer of two oxides. The use of different oxides impairs the optical properties, in particular the gloss of the pearlescent pigments. This can have a particularly detrimental effect if optically very high-quality pearlescent pigments are present, as are available nowadays as a result of the use of synthetic substrates. These relatively complex layer systems of several oxides or mixed oxides are also laborious to produce. There is an optimum pH range for the precipitation of each metal hydroxide or hydrated metal oxide. Mixed precipitations of various hydroxides or hydrated oxides therefore usually take place at pH values which represent a compromise between the optimum values for the precipitation of the pure hydroxides and that of the pure hydrated oxides. Therefore, as a rule no precipitation reactions are possible in which the conditions for the precipitation of all the inorganic components involved are optimally set in each case.
In EP 1 084 198 B1 effect pigments are described which exhibit very good adhesion to the base coat because of their surface modification with reactive orientation agents. However, EP 1 084 198 B1 discloses no weather- and UV-resistant pearlescent pigments.
The subsequent coating of pearlescent pigments with cerium hydroxide and SiO2 is described in M. Jäger, U. Schmidt, “Die Barriere macht den Unterschied”, Farbe and Lack August 2007, pp. 20-25. Pearlescent pigments with such a subsequent coating, unlike pearlescent pigments that have either only a cerium hydroxide or only an SiO2 layer, are characterized by their good weather resistance.